The present invention relates to a novel photoresist composition with very much reduced halation in exposure to light and capable of giving high residual film ratio with its performance retained even by a treatment at an elevated temperature.
Various kinds of electronic devices such as transistors, ICs, LSIs and the like are usually manufactured by a fine working technique utilizing a process of photoetching. The process of photoetching is typically performed by providing a layer of a photoresist composition on the surface of a wafer of silicon semiconductor, overlaying a mask of desired patterns on the layer of the photoresist composition, subjecting the layer of the photoresist composition to exposure to light through the mask followed by developing and etching to form the patterns of the photoresist composition and effecting a selective diffusion. In most cases, several times of the above process are repeated each with the step of selective diffusion whereupon electrodes and wiring connections are provided with aluminum to give the finished electronic device.
When the surface of the substrate has a high reflectivity as in aluminum or silicon in the above described procedures, difficulties are encountered in obtaining a high fidelity in the exposure of the layer of the photoresist composition to light through a mask bearing very fine patterns since the incident light is reflected irregularly by the substrate surface so that the portions of the photoresist not to be exposed to light are exposed to the irregularly reflected light. As a consequence, the line width of the patterns in the photoresist layer is broader than that of the patterns in the mask when the photoresist composition is of a negative type while, on the other hand, the former is thinner than the latter when the photoresist composition is of a positive type. This problem of low fidelity in the patterns is more serious on rugged surfaces having step-wise height difference after several times of the selective diffusion as described above.
The above problem is partly overcome by incorporating a photoextinction agent such as a dyestuff known by the trivial name of Oil Yellow which serves to reduce halation. On the other hand, conventional photoresist compositions with a cyclized rubber as the base ingredient are usually admixed with a bisazide compound such as 2,6-di(4'-azidobenzylidene)-4-methylcyclohexanone. When such a photoresist composition is exposed to ultraviolet light from an ultra-high pressure mercury lamp which emits line spectra mainly at 435 nm, 405 nm and 365 nm as the light source, the bisazide compound contained in the photoresist composition is decomposed by the ultraviolet light to form crosslinks resulting in disappearance of the original absorption at about 360 nm with consequent appearance of a new absorption at about 405 nm. Therefore, no satisfactory prevention of halation can be obtained by formulating a photoextinction agent for the light with the wavelengths of 435 nm, 405 nm and 365 nm from an ultra-high pressure mercury lamp resulting only in the decrease of the effective dose of the light.